There is known a microwave stripline filter in which it comprises a pair of dielectric ceramic substrates between which one or more resonator conductors are disposed in a predetermined pattern to form a resonator. In such a microwave stripline filter, one or more resonator conductors having a length corresponding to a predetermined resonant frequency are provided on each of the opposite inner surfaces of the paired dielectric ceramic substrates, a ground conductor is provided on the outer surface of the each dielectric ceramic substrate, and the dielectric ceramic substrates are superimposed and fixed to each other so that the resonator conductors provided on the inner surfaces thereof are integrally connected to each other. Such a stripline filter for microwaves is disclosed in U.S. Pat. Nos. 4,157,517 and 4,266,206 for example. Similar stripline filters can also be found in U.S. Pat. No. 4,785,271 and Japanese Patent Prepublication No. 62-263702.
With the microwave stripline filter of the abovementioned type, generally, each dielectric ceramic substrate is made of ceramic material such as BaO-TiO.sub.2, BaO-TiO.sub.2 -rare earth or the like.
However, there is disadvantage that the commonly used ceramic material has a resonant frequency which is decreased as the temperature is risen because the temperature coefficient of the resonant frequency is of a negative characteristic.
It is therefore an object of the present invention to provide a stripline filter for microwaves wherein the disadvantage in the conventional filter can be overcome, any variation in temperature which may occur during the operation of the filter can be compensated so as to reduce the variation of a resonant frequency.